1. Field of the Invention
The present invention relates to a color CRT, and in particular to an electron gun for a color CRT.
2. Description of the Prior Art
In general, a color CRT is a display used for a television, an oscilloscope, an observation radar, etc., and it displays an image on the front surface of a panel by controlling an electron beam from an electron gun according to a received image signal and by hitting a phosphor formed at the rear of the panel.
FIG. 1 is a schematic view illustrating a general CRT. The CRT includes a panel 102 as a front glass; a funnel 103 as a rear glass forming a vacuum space by being combined with the panel; a phosphor screen coated with a phosphor on the internal surface of the panel 102 for emitting light when struck by an electron beam; an electron gun 106 for emitting an electron beam 107 striking the phosphor screen 104; a deflection yoke 121 installed at a position separated a certain interval from the outer circumference of the funnel 103 in order to deflect the electron beam 107 toward the phosphor screen 104; a shadow mask 105 installed with a certain distance from the phosphor screen 104; a mask frame 109 for fixing/supporting the shadow mask 105; and an inner shield 110 installed a long toward the funnel 103 in order to prevent color purity deterioration by shielding external terrestrial magnetic fields.
As depicted in FIG. 2, the electron gun 106 includes a triode unit consisting of a cathode 130 arranged in a line and generating the electron beam 107 by heating an internal heater, a control grid 131 and an acceleration grid 132 for controlling and accelerating electrons from the cathode 130; and a main focusing lens unit consisting of a focusing grid 133 and an anode 135 for focusing and accelerating the electron beam generated from the triode unit.
The acceleration grid 132 may include a first acceleration grid 132a and a second acceleration grid 132b installed a certain distance from the control grid 131 and installed a certain distance from the cathode 130 towards the anode 135.
In general, the focusing grid 133 may include two to four grids, as depicted in FIG. 2. It includes a first focusing grid 133a installed between the first acceleration grid 132a and the second acceleration grid 132b; and a second focusing grid 133b installed with a certain distance from the second acceleration grid 132b. 
In the above-described electron gun 102, when power is applied, an electron beam is generated from the surface of the cathode 130 by heating of the heater, is controlled by the control grid 131, is accelerated by the first and second acceleration grids 132a, 132b, and is focused or accelerated by the first and second focusing grids 133a, 133b and the anode 135. The electron beam focused and accelerated by the focusing grid 133 and the anode 135 is deflected by the deflection yoke 121, and it is emitted to the phosphor screen 104 of the panel 102.
Herein, the control grid 131 is grounded, 500V˜1000V is applied to the acceleration grid 132, high voltage as 25 kV˜35 kV is applied to the anode 135, and an intermediate voltage as 20˜30% of an anode voltage is applied to the focusing grid 133.
In particular, because an electrostatic lens is formed between the second focusing grid 133b and the anode 135, the electron beam 107 generated in the triode unit is focused at the center of the phosphor screen 104.
The focusing state of the electron beam 107 can be described by Equation 1:Ds=√{square root over ((Dx+Dsa)2+(Dsc)2)}{square root over ((Dx+Dsa)2+(Dsc)2)}  (Equation 1)Where,    Ds: size of the final pixel    Dx: magnification of a main lens    Dsa: spherical aberration    Dsc: enlarged element by space charge repulsive effect
As shown in Equation 1, the size of the final pixel (Ds) on the screen is affected by a spherical aberration (Dsa). The main lens directly related to the spherical aberration (Dsa) is formed between the second focusing grid 133b and the anode 135. The corresponding holes 150, 160 are respectively formed at the second focusing grid 133b and the anode 135 so as to face each other. The corresponding hole 150 has an oval shaped rim structure, and the red, green, blue electron beams pass through the hole 150 at the same time.
An electrostatic screen grid 134 is formed at the corresponding holes 150, 160 as an inner grid. An inner grid formed in the second focusing grid 133b is called a first electrostatic screen grid 134a, and an inner grid formed in the anode 135 is called a second electrostatic screen grid 134b. The first and second electrostatic screen grids 134a, 134b are formed in order to have uniformity of the three (R, G, B) electron beams, and they make the three electron beams have the same shape.
As depicted in FIG. 3, in the first and second electrostatic screen grid 134a, 134b, three electron beam through holes 140 arranged in a line are formed so as to pass three electron beams, and the three electron beams through holes 140 and the corresponding holes 150, 160 form the main focusing lens.
In a conventional electron gun 106, the first and second electrostatic screen grids 134a, 134b have the same shape and size, the distance (Lb1) between the first electrostatic screen grid 134a and the corresponding hole 150 is same as the distance (Lb2) between the second electrostatic screen grid 134b and the corresponding hole 160.
In addition, as depicted in FIG. 4, the three electron beam through holes 140 formed at the first and second electrostatic screen grids 134a, 134b consist of two external holes 140a and one central hole 140b. Herein, the external hole 140a has a vertical size (WO) greater than a horizontal size (HLO+HRO), and generally it has a shape that is longer in the vertical direction. FIG. 4 shows the shape of the electron beam through hole of the conventional electrostatic screen grid 134. The center of the hole is the central point of a vertical line traversing the largest vertical extent of external hole 140a. In the horizontal direction, the distance from the center of the external hole 140a to the left and right sides of the central hole 140b are the distances HLO and HRO respectively. The horizontal size of the external hole 140a can be described as HRO+HLO.
In the conventional electron gun, HRO of the external hole 140a is 2.53 mm, and HLO is 2.90 mm resulting in a horizontal size of 5.43 mm. The vertical size of the external hole 140a is 5.96 mm, and accordingly it has a vertically long shape.
The electron beam convergence is defined as the distance between the red (R) electron beam and the blue (B) electron beam among three electron beams on the screen. As depicted in FIG. 4, in the conventional electron gun 106, the distances between the external hole 140a and the central hole 140b is generally 5.5 mm. The distance between the red (R) electron beam and the blue (B) electron beam is 2×S, and the electron beam convergence is about 11 mm in the conventional electron gun.
In the first and second electrostatic screen grids 134a, 134b, the red electron beam is separated from the blue electron beam by 11 mm, and the distance is about 8–10 mm on the screen. However, it has to be “0” on the screen in order to prevent pixel distortion. Generally, only when the electron beam convergence (OCV) is within 2 mm on the screen, is it possible to adjust. Accordingly, in the conventional art, in order to solve this problem, a pre-convergence is performed between the first accelerating grid 132a and the first focusing grid 133a, and accordingly the electron beam 107 passes the grids from the first focusing grid 133a to the main lens having a potential difference different from each other. However, when the electron beam 107 passes the control grid 131 and the second focusing grid 133b, the electron beam convergence of the first and second electrostatic screen grids 134a, 134b having almost same shape and size is lowered, and accordingly it exceeds the adjustment range.